The tumor microenvironment consists of numerous stromal cell types whose diverse functions foster malignancy. Tumor-associated myeloid cells, including macrophages, have been implicated in promoting tumor growth and tumor immune evasion, and are associated with poor prognosis in cancer patients. However, the differentiation pathway employed by these cells and their mechanisms of immune suppression are largely uncharacterized. Using an autochthonous mammary tumor model, we have recently found that tumor growth induces the differentiation of a unique subset of tumor-associated macrophages (TAMs) from inflammatory monocytes. Gene expression profiling studies reveal that compared to mammary tissue macrophages (MTMs), TAMs acquire a Notch-dependent gene expression signature. Genetic ablation of the key transcriptional regulator of the Notch pathway, RBPJ, results in a selective loss of TAMs and low tumor burden concomitant with the restoration of tumor-infiltrating cytotoxic T cell responses. In this application, we will use loss- and gain-of-function mouse models to determine the exact Notch receptor(s) and Notch ligand(s) involved in TAM differentiation, and to explore whether the expression of a constitutively active form of Notch activates the TAM differentiation program in non-tumor bearing mice. Furthermore, the precise Notch- dependent gene expression program in TAMs will be elucidated. Compared to MTMs, TAMs express high levels of the av8 integrin. In autoimmune disease models, the av8 integrin expressed by myeloid cells is essential for the induction of T cell tolerance via the activation of the immunosuppressive cytokine TGF-1. Using a T cell culture system, we will investigate whether TAMs function as antigen-presenting cells, and suppress T cell responses by engaging the TGF- pathway. In addition, the definitive function of TAM- expressed 8 integrin in the control of T cell tolerance to tumors will be determined with a strain of myeloid cell- specific Itgb8-deficient mice. Completion of these studies will not only illuminate the natur of tumor-elicited inflammatory responses but also provide insights into the therapeutic intervention in human breast cancer and perhaps other cancers of epithelial cell origin.